Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

The paper then goes on to analyze O2s (shallow core levels), taken at different photon energies, and, again assign them to different O atoms according to PDOS calculafions.This makes a liftle bit more sense, but I am doubfful that the rather pronounced changes in the core levels would only be due to IMFP length effects, as claimed.Then this should lead to a rather smooth change, which should not be very pronounced in the hv range considered (39 to 200 eV).
The observed relafively drasfic intensity changes could be due to two phenomena: 1) hybridizafion with Ti band (and the well-known resonant photoemission effect) and/or 2) if taken with the energy analyzer set to a narrow angular acceptance range, photoelectron diffracfion.
As far as I can see, neither of these effects has been considered.
The analysis of the Ti3p core level spectra and the assignment of the defect state to different O vacancies makes sense only if the excess electrons are in a small polaron state right next to the vacancy.This is possibly true, but more work would be needed to show this.
Summarizing, the interpretafion of the experimental data is naïve at best, and completely wrong at worst.

I suggest rejecfing this paper.
Reviewer #4 (Remarks to the Author): The paper by Ma et al. reports on the assignment of different coordinafion structures for anatase TiO2(001) with their corresponding electronic structures.The work is a mulfi-technique experimental approach combined with DFT calculafions.While STM images provide informafion about the coexistence of two specific coordinafion environments at the ridge sites, with ARPES and XPS, the authors get insights into the electronic structure.However, it is the combinafion of these experimental techniques with theorefical DFT calculafions that allows the determinafion of seven local coordinafion environments in the anatase TiO2(001)-(1x4) surface reconstrucfion.The subject of this research is of great interest due to the relevance of this material, TiO2, in different the technological applicafions, being of special importance in catalysis due to its surface reacfivity.The manuscript achieves a high enough scienfific ranking to be accepted in Nature Communicafion.The authors' experiments are well done and well thought out and theorefical calculafions are fundamental in assigning the different electronic states to the coordinafion environments.The work not only demonstrates the different coordinafion structures of the anatase TiO2(001)-1x4 surface, but also provides a paradigm to explore the structure and electronic properfies of TMOs.However, there are some minor point that the authors need to address: -Catalysis and photocatalysis are not intrinsic properfies of materials per se, but rather phenomena that arise from the interacfion between materials and chemical reacfions (and light in the case of photocatalysis).Therefore, it is inaccurate for the author to state that TMOs "exhibit versafile funcfional properfies such as catalysis and photocatalysis" being more appropriate that TMOs possess versafile funcfional properfies suitable for various applicafions, including catalysis and photocatalysis.Similarly, strong correlafion is not a property but a phenomenon.The sentence "These fascinafing properfies ranging from strong correlafion to surface catalysis …" is not enfirely correct.More precisely, the authors may refer to "These fascinafing phenomena ranging from strong correlafion to surface catalysis …".
-In the case of the O-2s spectra fits (Fig. 3b-c), are the minimum four subspectra needed to match the XPS O-2s curve?Would it be possible to achieve a good fit with fewer curves?Clarify in the manuscript.
-The metallic state in the XPS Ti-3d spectra (Fig. 4) appears already after 1 min of light irradiafion and remains almost constant for longer.Comment on it.
-In this work, the assignment of the two gap states in the Ti-3d XPS spectra, which were previously observed but not understood unfil now, is quite important.However, the Ovridge GS has two peaks and, although they are centered at 1.1 eV, one is located at 1.6 eV.This BE value is the same as the GS2 contribufion.Could it interfere with the GS assignment?Add some discussion in the manuscript to clarify this point.
-Several sentences in the manuscript suffer from clarity and grammar issues.The manuscript requires improvement to achieve linguisfic correcfion.
This interesting work reports the determination of distinct coordination environments of the prototypical binary oxide anatase-TiO2(001)-(1×4).The integration of momentum, energy, and real-space resolutions in O 2p, O 2s, and Ti 3d spectra, coupled with scanning tunneling microscopy and density functional theory, constitutes a significant advancement in surface science and catalysis.However, the reliability of this innovative approach necessitates further validation using reference systems, preferably simpler ones with fewer complex coordination environments.The current limitations of the approach are evident in the results and conclusions, highlighting the need for broader applicability.Additionally, several issues must be addressed before considering the publication of this work: Author reply: We thank the reviewer for finding our study interesting.Further validation of our approach using a simpler reference system is a good suggestion.We made comparison with the results from the simpler rutile-TiO2( 110)-(1×1) surface.
So far, we are aware of few reports about the O 2s features in anatase-TiO2(001)-(1×4).As suggested by the reviewer, this comparison is certainly more helpful to draw attention on the multiple peaks in anatase-TiO2(001)-(1×4) surface.We thank the reviewer for the insightful suggestion.As for the Ti 3d gap state in the rutile-TiO2( 110)-(1×1) surface, it commonly presents as a gap state at 0.9 eV below Fermi level (EF), which has been assigned to the surface oxygen defects and/or interstitial Ti atoms [Yim et al., Phys. Rev. Lett. 104, 036806 (2010); Wendt et al., Science 320, 1755-1759(2008)].This is in sharp contrast to the two gap states observed in the anatase-TiO2(001)-(1×4) surface.
In summary, we have done XPS measurements using a simpler and relevant rutile-TiO2(110)-(1×1) surface as a reference, as suggested by the reviewer.It is found that the coordination induced differences are small, and the electronic structures are less complex in the rutile-TiO2(110)-(1×1) surface.The comparison between rutile-TiO2( 110)-(1×1) and anatase-TiO2(001)-(1×4) surfaces does give fruitful insight into the effects of the diverse coordination environments in reconstructed anatase-TiO2(001)-(1×4) surface, which are demanded to be clarified.Our approach in this manuscript does point to the goal.We have added the data in Supplementary Fig. S5a,b We have added these results as new data in Fig. 1d-e and the corresponding description in this revised manuscript (page 6) to illustrate the distinct contrast in qPlus-AFM image could be evidence for the existence of ADM and AOM configurations, it reads We perform in situ STM and AFM experiments to characterize the structures at a high resolution.The empty-state STM image mainly integrate the Ti electronic state on the ridge and terrace (Fig. 1d, left panel), while the AFM image (given in inverted contrast because of attraction force regime in imaging) mainly reflect the contrasts of O atoms along the ridges (Fig. 1d, right panel).The two groups of O atoms can be distinguished from their relative contrasts, as labeled by ADM and AOM according to the relative heights in their structural models 32,38 (Fig. 1e).Both of the STM and the AFM images indicate the coexistence of the ADM and AOM structures on the ridge.
2. For the 1:1 mixed ADM-AOM structure, there might be another setup where two AOM patterns are together.Can its energy related to the oxygen chemical potential be added to Figure S6?Also, it would help if the formula used to calculate the phase diagrams for the different ADM-AOM structures is provided.
Author reply: Thanks for the suggestion.We have added this setup where two AOM patterns are together to Supplementary Fig. S9 (also shown below in Response Fig. 3).
Here, we have added the corresponding calculation formula in Methods of the revised manuscript for the calculations of the surface energies and phase diagrams of the mixed ADM-AOM structures.3. Given the many ways Ti 3+ can be arranged near the Ov site, it's essential to look at all these configurations to determine the most stable configuration of the defective anatase-TiO2(001).It's not clear if the authors used spin-polarized calculations during the structural relaxation processes for the defective anatase-TiO2(001) or if it was only used for single point electronic structure calculations.
Author reply: We thank the reviewer for this suggestion.As it is noticed, ref. 40 [Bigi et al., Phys. Rev. Mater. 4, 025801 (2020), Fig. 6] has systematically studied the possible configurations of OV defects at anatase-TiO2(001)-(1×4).They also found the adsorption of O2 can quench the gap states (GSs), and thus assigned the origin of GSs to the surface OV defects.
Based on such valuable information, we considered four kinds of possible OV configurations at surface, as labeled by OV-1 (OV-RIDGE), OV-2 (OV-SIDE), OV-3 (OV-3C) and OV-4 (OV-TERRACE) in Response Fig. 4a.The formation energies of the OV configurations are calculated by PBE functional with spin-polarized calculations and listed in Response Fig. 4b.It can be seen that the OV-1 (OV-RIDGE) is the most likely to appear at ridge sites, and the OV-3 (OV-3C) and OV-4 (OV-TERRACE) with energy difference of 0.6 eV may co-exist at terrace sites.
In order to calculate the GSs more accurately, we performed the spin-polarized DFT calculation during the structural relaxation processes with PBE+U functional (U = 3.9 eV).The re-calculated DOSs for OV-1 (OV-RIDGE), OV-3 (OV-3C) and OV-4 (OV-TERRACE) sites are shown in Response Fig. 5.The antiferromagnetic state (Response Fig. 5a-c) has a lower formation energy than the ferromagnetic (FM) state (Response Fig. 5d-f).
The excess electrons are mainly distributed in adjacent Ti atoms, reducing the Ti 4+ to Ti 3+ , implying the formation of a small electron polaron [J.(d-f).DOS of 1-3 layers is extracted to eliminate the effects of (1×1) surface at the last layer, the calculated bandgap is estimated to ~ 2.5 eV.Charge density contours of the excess electron states induced by OV defects.The excess electrons are mainly distributed in adjacent Ti atoms, reducing the Ti 4+ to Ti 3+ , implying the formation of a small electron polaron.
To address comments 3, we have added Response Fig. 4 and Fig. 5 to Supplementary Fig. S11 and Fig. S12, respectively.The antiferromagnetic results in Response Fig. 5 are merged into Figure 4e,f in the revised manuscript.We have added a corresponding discussion in this revised manuscript on pages 14-15, it reads We calculate the configurations and formation energies for possible surface OV defects at ridge and terrace sites using spin-polarized DFT (Supplementary Fig. S11).From the formation energies, it is found that the missing of one OTOP is the most possible OV defect at ridge (OV-RIDGE, Fig. 4e), and the missing of a bridging O atom along [100] direction or along [010] direction could be the OV defect at terrace (OV-3C and OV-TERRACE, Fig. 4f).The calculated electronic structures with PBE+U functional (U = 3.9 eV) 40 show that each OV defect can contribute to a GS with its excess electron redistributing to adjacent Ti atoms (Fig. 4e-f).The charge redistribution and the distortion of the lattice in the vicinity imply the formation of small electron polaron, similar as the small polaron of OV at rutile-TiO2(110) surface 57 .But, from the pDOS with either antiferromagnetic 58 or ferromagnetic states (Supplementary Fig. S12), the energies of different OV defects are not separated clearly, making the assignment difficult.
4. The authors have given theoretical proof for the presence of both ADM and AOM structures.But is there direct proof for both defective structures being present at the same time?What are the formation energies for these two kinds of oxygen vacancies?
Author reply: (1) After performing the NC-AFM experiments (Response Fig. 2), we think the two distinct contrasts in the qPlus-AFM image could be direct evidence for the presence of alternated ADM and AOM structures in real-space.Also, another evidence could be found in O 2s spectra the Fig. 3f in the main text, in which the ADM ratio is increasing and the AOM ratio is decreasing by controlling the surface reduction.
(2) The AOM is the fully oxidized form with one added O atom to the ADM structures in each lattice.Under reduction, the AOM loses one O to ADM, and then loses the second one to form a defective OV-RIDGE.Therefore, the defective OV-RIDGE structures of AOM and ADM are indeed the same, and it is not expected to observe any difference from them.Here, to investigate the multiple OV, we adopt the same kind (Response Fig. 6a-e) or different kinds of OV (Response Fig. 6f-i) to construct the OV pairs below: 1) same kind of OV pairs.Response Fig. 6a-e show the OV pairs consisting of two neighboring OV defects.It can be found that the OV-(1,1) pair at ridge sites has the lowest formation energy of 8.7 eV (Response Fig. 6a).Experimentally, the intensity of GS1 can continue to increase while that of GS2 quickly achieve saturation (Fig. 4d in the main text), suggesting that GS1 related defects can be reasonably assigned to ridge sites.However, it is 0.7 eV higher than the sum of the formation energies of two separated OV-1.Thus, it suggests such OV pairs might be quite unstable.This is in line with the previous studies at rutile-TiO2( 110)-(1×1) surface that the OV pairs are thermodynamically unstable with repulsion to dissociate into separated single OV's, as reported by our group [Cui et al., J. Chem. Phys. 129, 044703 (2008)] and other group [Zhang et al., Phys. Rev. Lett. 99, 126105 (2007)].
2) different kinds of OV pairs.Response Fig. 6f-i show the OV pairs consisting of two different kinds of separated single OV's.The OV-(1,3) and OV-(1,4) pairs have the lower formation energy.We further compare the electronic properties of multiple Ov's, for example OV-(1,3,4), as shown in Response Fig. 7.We find that the charge density contours and configurations are basically a direct superposition of separated single OV features, the GSs show very close energies within the underestimated bandgap of 2.5 eV (Response Fig. 5).The antiferromagnetic state (Response Fig. 7a) has a lower formation energy than the ferromagnetic (FM) state (Response Fig. 7b).To address this comment 5, we have added the Response Fig. 6 to Supplementary Fig. S13.The antiferromagnetic results in Response Fig. 7 are merged into Figure 4g in the revised manuscript.We have added some sentences in this revised manuscript on page 15, it reads Such energy inaccuracy is possibly because the self-interaction error in DFT, which usually leads to an underestimated bandgap of TiO2.In particular, when multiple OV defects are considered (Supplementary Fig. S13), the GSs show very close energies within the underestimated bandgap of 2.5 eV (Fig. 4g).

Reviewer #2 (Remarks to the Author):
This is an interesting paper, although I do not consider it as a major advance.The authors certainly oversell the work.There is also a major need for extensive correction of the grammar and English which is quite bad in many places.
Author reply: We thank the reviewer for finding our manuscript interesting.TiO2 seems as a simple binary oxide, but the existing complicated structures and electronic states have not been well understood.In particular, different reconstructed surfaces may appear and play important roles on many physical and chemical processes.Here, our study, using the prototypical surface of anatase-TiO2( 001)-(1×4), demonstrates how the multi-aspect information can be obtained and used to univocally determine the structure-property relationships.
Identification of the surface electronic states is of importance in many surface-related processes of TiO2.To establish the intricated coordination environment of O atoms with the electronic structures as possible as we can, our approach included the characterization using microscopic and spectroscopic techniques together.Our results may give insights into several aspects, as: (1) the identification of O 2p surface states from the bulk ones using momentum-resolved band structure from ARPES; (2) identification of the surface states due to existing of ADM and AOM surface structures from the O 2s spectra; (3) identification of gap states from OV's at ridge and the terrace sites.These aspects have not been clearly documented before.As an important but intricated reconstructed surface, obtaining such global parameters provides insightful understanding, and represents a major advance.We believe our findings will benefit to a wide range of readers.
We have revised the grammar and English as suggested by this reviewer, and also revised the manuscript substantially following the comments by other reviewers.We appreciate if this reviewer may find our revised manuscript has been improved.

I have a few comments:
Abstract needs work: The sentence "We resolve each…" needs rewriting.(2) We further consider the resonant photoemission effect.It has been shown that a resonance process can occur at around hυ ~ 47 eV for Ti 3p→3d optical transition at anatase-TiO2(001) surface (refs. 53,54) [Thomas et al., PRB 67, 035110 (2003); PRB 75, 035105 ( 2007)].It is seen that the bulk peak at 22.7 eV is monotonously increasing from low-to-high hυ, and becomes dominant at hυ > 120 eV (Fig. 3a in the main text), indicating the bulk peak intensity is mostly related to the electron inelastic mean free path (IMFP).Here, we have done more experiments using variable hυ (39-55 eV) excitations.The hυ-dependent data are given below in Response Fig. 9c (also added in Supplementary Fig. S5).It is seen that the surface peaks (in the range of about 18-22 eV) show a maximum intensity at about hυ ~ 43 eV (red arrow), and a local maximum intensity at about hυ ~ 46 eV (black arrow) for the bulk one.The appearance of the maximum intensity could be from the resonance by hybridization with Ti band.
Even though, the obvious different behaviors from the different peaks (as the labeled bulk and surface states) do indicate their different origins, which may further rationalize our analysis.
Therefore, we can see that we may still separate the surface states from the bulk ones, even the resonant photoemission effect may occur.
(3) As for the possible effect of photoelectron diffraction, we can exclude this possibility in our experiment.The reviewer mainly concerned the possible enhancements of the ARPES signals at certain angles, because of the diffraction of photoelectrons by the surface atoms (lattice).Considering the excitation photon energy of hυ = 40-55eV, the kinetic energy (Ekin) of the outgoing photoelectrons at (001) surface is about 12-27 eV, which falls in the range of those O 2s signals that we discussed.In this kinetic energy range, the photoelectron wavelength is in the range of λ = 2.36 ~ 3.54 Å.Using 2dsin = λ, we get the estimated angles  = 19~28º for the first-order diffraction peak using the unconstructed lattice constant of 3.80 Å.These angles are obviously larger than the incident angle limit of 15 of the slit used in the ARPES.However, this may not totally exclude the possible diffraction from the (14) reconstructed surface structure.
As a further confirmation, we made additional analyses using the angle distribution curves (ADCs) and the energy distribution curves (EDCs).As shown in Response Fig. 10, such analyses provide more evidence to exclude the possibility that the photoelectron diffraction may affect the O 2s spectra by the reconstructed surface structure.Response Fig. 10a-c show the ARPES spectra measured at the excitation photon energies of 39, 43 and 46 eV, respectively.
The ADCs, obtained correspondingly by integrating the signals within the Eb range of 18~24 eV of each ARPES spectrum, are superimposed on Response Fig. 10a-c.It is seen that the ADCs at each photon energy give overall Gaussianlike shape for the integrated intensity distributions against θ, obviously no diffraction-enhanced intensity with 15.Moreover, Response Fig. 10d-f  To address this comment, we have added the Response Fig. 9 to Supplementary Fig. S5, the Response Fig. 10 to Supplementary Fig. S7 and added this discussion in the revised manuscript on page 11, it reads We can exclude the effect of the photoelectron diffraction due to the possible diffraction-caused enhanced angle-dependent intensity variations (Supplementary Fig. S7).The angle distribution curves (ADCs) at each photon energy give overall Gaussianlike shape for the integrated intensity distributions against θ, obviously no diffractionenhanced intensity with 15.The EDCs show nearly the same feature at each excitation photon energy, showing the angle-independent O 2s spectra.These analyses can exclude the possibility of the diffraction effect by the reconstructed surface structure.While, by measuring the spectra using the tunable excitation phonon energy in the range of hυ ~ 39 -55 eV , the resonant photoemission processes were observed to occur at around hυ ~ 43 -46 eV (Supplementary Fig. S5c), which could be assigned to the Ti 3p→3d optical transition at anatase-TiO2(001) surface 53,54 .Such resonant photoemission processes could enhance the photoemission intensities of the surface semi-core levels at certain excitation photon energy, and make the peaks more distinguishable in the O 2s spectra.Nevertheless, this effect does not obviously contribute any additional peak and much easily be recognized according to the analysis of our results.
The analysis of the Ti3p core level spectra and the assignment of the defect state to different O vacancies makes sense only if the excess electrons are in a small polaron state right next to the vacancy.This is possibly true, but more work would be needed to show this.However, there are some minor points that the authors need to address: Author reply: We thank the reviewer for the insightful comments and finding our results interesting.We have addressed all the comments and revised the manuscript accordingly.
-Catalysis and photocatalysis are not intrinsic properties of materials per se, but rather phenomena that arise from the interaction between materials and chemical reactions (and light in the case of photocatalysis).Therefore, it is inaccurate for the author to state that TMOs "exhibit versatile functional properties such as catalysis and photocatalysis" being more appropriate that TMOs possess versatile functional properties suitable for various applications, including catalysis and photocatalysis.Similarly, strong correlation is not a property but a phenomenon.The sentence "These fascinating properties ranging from strong correlation to surface catalysis …" is not entirely correct.
More precisely, the authors may refer to "These fascinating phenomena ranging from strong correlation to surface catalysis …".
Author reply: We thanks the reviewer for the kind correction of the descriptions.We have revised all these descriptions.
Author reply: Thanks for point out these typos.We have revised.
-In the case of the O-2s spectra fits (Fig. 3b-c), are the minimum four sub-spectra needed to match the XPS O-2s curve?Would it be possible to achieve a good fit with fewer curves?Clarify in the manuscript.

Author reply:
We agree that the multi-peak fitting in XPS spectrum is tricky.We did this carefully by taking account of: first, the kinks and shoulders in the spectrum could roughly point to the possible peaks, as shown by the green arrows in the raw spectra and the corresponding spectra after background subtraction (Response Fig. 11a,b); second, in a series of spectra with different excitation hυ, the corresponding peaks should keep at the same energy, but change in intensity, as labeled by the dashed lines (Response Fig. 11c).
As suggested, we also tried a three-peak fit for the same spectra in Response Fig. 11d.
It can be found that the middle peak (blue shade) cannot well reproduce the two kinks at 21.2 and 20.1 eV, and the energies of middle (blue shade) and right (brown shade) peaks are changing at different hυ.Thus, the three-peak fit is apparently worse than the four-peak fit.
To address this comment, we have added the Response Fig. 11 to Supplementary Fig. S6 with these fitting details. -The metallic state in the XPS Ti-3d spectra (Fig. 4) appears already after 1 min of light irradiation and remains almost constant for longer.Comment on it.
Author reply: Thanks for the good question.This is because the metallic state (MS) is not a single peak, but an electron pocket evolving below EF.The MS is formed by the excess electron doping from OV defects, which causes the band bending and thus drags the conduction band minimum below EF to form an electron pocket.This had been investigated in our previous study (ref. 39) [Nano Lett. 21, 430-436 (2021)] and other group 's study (ref. 40) [Bigi et al., Phys. Rev. Mater. 4, 025801 (2020) -In this work, the assignment of the two gap states in the Ti-3d XPS spectra, which were previously observed but not understood until now, is quite important.However, the Ov-ridge GS has two peaks and, although they are centered at 1.1 eV, one is located at 1.6 eV.This BE value is the same as the GS2 contribution.Could it interfere with the
, for comparing O 2s spectra from rutile TiO2(110)-(11) and anatase TiO2(001)-(14) surfaces.1. Inferring the existence of ADM and AOM configurations solely from the STM image is initially imprecise.STM image contrasts can stem from various sources, and deducing the presence of ADM and AOM solely based on one image is not entirely convincing.While the subsequent sections of the article aim to validate the existence of these structures through spectroscopy, robust experimental evidence remains incomplete.Author reply: We agree with the reviewer.To address this comment, we have performed in situ measurements using integrated STM and non-contact atomic force microscopy (NC-AFM) with the same tip mounted on a qPlus sensor.The results are given below in Response Fig. 2. The nonuniform STM features on the ridge have been correspondingly resolved by the AFM, under either constant height or constant force modes.It can be seen that although STM gives a blurry nonuniform contrast at the ridge (Response Fig. 2a, left panel), the high-resolution NC-AFM image show separated spots (Response Fig. 2a, right panel).The spots have two distinct widths and heights as shown in the corresponding line profiles (Response Fig. 2b).Because the AOM involves one more O atom that can increase the repulsive force, we assign the brighter spots to AOM structures and the dimmer one to ADM structures in the AFM image.Therefore, by comparing the same area STM and NC-AFM images, we suggest that the nonuniform STM contrast indeed imply the presence of alternate AOM and ADM structures.Response Fig. 2. (a) A set of in-situ empty-state STM (1.5 V, 10 pA) and the NC-AFM images at a frequency shift of −29 Hz within the same area, measured at 5 K, using a W tip. (b) Line profiles extracted from the corresponding colored lines in (a).The line profile from STM image (red curve) also shows the nonuniform electronic distributions, and the one from AFM (blue curve) present two distinct contrasts, labeled as ADM and AOM, according to the relative height in their models.

Response Fig. 3 .
The stabilities and phase diagrams of the mixed ADM-AOM structures.(a)The optimized structures of pure ADM, 3:1 (the numbers denote the ADM:AOM ratio); 1:1, 1:3 and pure AOM.The surface O atoms of OTOP, OSIDE and OTERRACE are colored by red, brown, and green, respectively.Although the two setups marked as 1:1-1 and 1:1-2 have different structures, they have nearly identical surface energies (difference within ~5 meV).(b) The calculated surface energies of the five structures in (a), plotted as a function of oxygen chemical potential Δ O for two different lattice constants of a = 3.8 Å (left panel) and a = 3.9 Å (right panel), respectively.A larger lattice constant a = 3.9 Å is used due to anatase-TiO2(001) thin films epitaxially grown on 0.7 wt% Nbdoped SrTiO3(001) substrates.(c) The corresponding phase diagrams with oxygen pressure and annealing temperature, calculated on the basis of the lattice constants of a = 3.8 Å (left panel) and a = 3.9 Å (right panel), respectively.
Phys.Chem.C 113, 14583-Response Fig. 4. Formation energies of possible surface OV configurations.(a) Side view of the (1 × 4) reconstructed anatase TiO2(001) slab model.All possible surface OV configurations are represented with circles.The atoms of OTOP, OSIDE, OTERRACE and OBULK are colored differently.(b) OV formation energies (eV) at different surface sites.The OV-1 (OV-RIDGE) is the most likely to appear at the ridge sites, and the OV-3 (OV-3C) and OV-4 (OV-TERRACE) with energy difference of 0.6 eV may co-exist at the terrace sites.(c-f) Relevant OV defect configurations for OV-1 (OV-RIDGE), OV-2 (OV-SIDE), OV-3 (OV-3C) and OV-4 (OV-TERRACE), respectively.OV sites are marked by black arrows.Response Fig. 5.The calculate the GSs of different OV sites with antiferromagnetic and ferromagnetic (FM) states.Calculated charge density contours of excess electrons and DOSs of total (gray) and 1-3 layer (yellow: spin up, blue: spin down) based on ADM model with an OV-1 (OV-RIDGE) at ridge (a,d), an OV-3 (OV-3C) at terrace (b,e) and an OV-4 (OV-TERRACE) at terrace (c,f), respectively.The antiferromagnetic state (a-c) has a lower formation energy than the ferromagnetic state

5.
The study seems to focus on single Ov.What happens when there are multiple Ov, in terms of shape and electric properties?How do these findings compare to what's seen in experiments?Author reply: As shown in Response Fig. 4 above, there are three possible kinds of single OV defects at [the OV-2 (OV-SIDE) has the highest energy and is not considered].
show the EDCs, which were obtained by cutting at the given angles (in each curve, the signals were integrated within 1 at the labeled angle), in comparison with the normalized total signals (integrated signals with the whole range of 15).It is obvious that the EDCs show nearly the same feature at each excitation photon energy, showing the angle-independent O 2s spectra.These analyses can exclude the possibility of the diffraction effect by the constructed surface structure.Therefore, on the basis of the distinguishable features from those of bulk ones, our observations of the multiple peaks of the O 2s spectra can be assigned to the diverse coordination environments of the surface atoms.Response Fig. 10.Angle -and energy-dependent O 2s spectra.(a-c) Three selected Eb-θ cuts with hυ = 39, 43 and 46 eV excitation.The angle distribution curves (ADCs) are marked by white lines (signal integration within the Eb range of 18~24 eV).(d-f) The corresponding energy resolution curves (EDCs) at different scattering angles (in each curve, the signals were integrated within 1 at the labeled angle), in comparison with the normalized total signals (integrated signals with the whole range of 15).It is obvious that the EDCs show nearly the same feature at each excitation photon energy, showing the angle-independent O 2s spectra.
The paper by Ma et al. reports on the assignment of different coordination structures for anatase TiO2(001) with their corresponding electronic structures.The work is a multi-technique experimental approach combined with DFT calculations.While STM images provide information about the coexistence of two specific coordination environments at the ridge sites, with ARPES and XPS, the authors get insights into the electronic structure.However, it is the combination of these experimental techniques with theoretical DFT calculations that allows the determination of seven local coordination environments in the anatase TiO2(001)-(1×4) surface reconstruction.The subject of this research is of great interest due to the relevance of this material, TiO2, in different the technological applications, being of special importance in catalysis due to its surface reactivity.The manuscript achieves a high enough scientific ranking to be accepted in Nature Communication.The authors' experiments are well done and well thought out and theoretical calculations are fundamental in assigning the different electronic states to the coordination environments.The work not only demonstrates the different coordination structures of the anatase TiO2(001)-1×4 surface, but also provides a paradigm to explore the structure and electronic properties of TMOs.

Response Fig. 11 .
Peak fitting of O 2s spectra.(a) The measured O 2s XPS of anatase-TiO2(001)-(1×4) excited by hυ = 39, 43 and 46 eV, respectively.The backgrounds (BGs) to be subtracted are shown with grey lines.(b) The corresponding spectra after BG subtraction.The green arrows point to possible peaks.Peak fitting of O 2s spectra for four-peak fit (c) and three-peak fit (d), respectively.(c) are same as Fig. 3b-d in the main text.
].With the high flux synchrotron light, 1 min irradiation can create enough OV defects to induce both the GSs and MS.With longer irradiation, the intensities of GSs increase gradually, while the intensity of MS looks almost constant.We use our previous data (Fig. S3 in ref. 39) to illustrate how the electron pocket of MS evolves under longer irradiation (Response Fig. 12).To observe an intact electron pocket, the data should be collected in the second Brillouin zone Γ 10 , because the matrix element effect causes the vanishing intensity in the first Brillouin zone Γ 00 .As shown in Response Fig. 12, with longer time irradiation, the intensity at EF remains almost unchanged, but the electron pocket (indicated by the white dashed curves) becomes larger with an increasing Fermi wavevector  F .The increasing electron density n under longer irradiation can be obtained by  =  F 3 /3 2 , as shown by the red numbers in Response Fig. 12a.Response Fig. 12 (adopted from ref. 39).Spectral evolution of the raw ARPES cuts (a0-e0), second derivative cuts (a1-e1) and EDCs at Γ10 point (a2-e2) with increasing carrier doping at anatase TiO2(001)-(1×4) surface by synchrotron irradiation.Synchrotron irradiation time: 2 min (a), 4 min (b), 8 min (c), 12 min (d) and 15 min (e).The white and yellow arrows point to the plasmonic polaron (PL) and kink structure, respectively.